The side chain signal in the lower half of the diagram is derived
from either the main stereo input (1 & 2) or an external stereo
input (3 & 4) as determined by the 'External' switch. It can then be
adjusted using the 'Trim' and 'Low Cut' controls before being used to
control the output signal.

The 'Listen' switch can
be used to temporarily divert the sidechain signal to the audio output
to allow the signal to be monitored, both aurally and via the input
level meter.